The wording on this one was weird. I couldn't figure out if B meant that there was a 50/50 chance of choosing defective/nondefective items or if it meant that the field inspector was unbiased in his choosing. Obviously there is not a 50/50 chance since the contract requires <5%. I ended up getting the right answer with D because it is a flaw and the sentence was not nearly as ambiguous. I initially circled B, but I starred the answer, had enough time to go back to check it, and realized D was correct.

The samples that the field inspectors selected have a 20% defectiveness rate.

The supplier is obligated to have only a 5% defectiveness rate.

The author of the stimulus concludes that, therefore, the supplier hasn't met the obligation.

However, what if the field inspectors purposely choose samples that are more likely than average to be defective? That would allow for the possibility that the samples they choose have a 20% defectiveness rate, while the overall defectiveness rate could still be less than 5%.

The answer is not B because the argument never claims that the field inspectors were equally likely to pick something defective as something not defective.

Someone else just asked me for an explanation of this one via PM, and I'll repost my reply here in case it may help anyone else:

59.2.20)

The inspectors sent samples from some of the manufacturer's locations. Testing showed that 20% of the samples were defective. Therefore, the manufacturer is in breach of the contract which requires a defect rate of 5% or below.

The assumption here is that the samples sent in for testing are representative of all the items manufactured. If they aren't a representative sample, we have no way to determine what percentage of the products in general are defective.

A) We don't know how large or small a sample is being tested; No.B) The argument doesn't make this presumption; in fact, if it was presumed that they were just as likely to pick a defective item as a good one, there should have been a 50% defect rate rather than 20%; No.C) We don't know how many sites were inspected, or how many sites there are altogether; No.D) If it's possible that the inspectors tried to deliberately choose defective samples, this would create a near-certainty that the samples selected are not representative of the whole; Yes.E) How many times the sites were visited is irrelevant; No.

I wavered between B and D on this one myself, too. Eventually I chose D because the argument clearly assumed that the sample was in fact representative, and therefore overlooked the possibility that it was not. I decided that this was a stronger answer than B; I was not able to satisfactorily eliminate B as an option until I reviewed the test later.